The critical immunological feature of visceral leishmaniasis is the complete absence of cell mediated immunity to leishmanial antigens. Patients have been shown to have negative intradermal skin tests to Leishmanin, absent lymphocyte blastogenesis, and decreased IL-2 and IFN- gamma production in response to parasite antigens. Experimental models of visceral leishmaniasis suggest that the disease is characterized not by the lack of cytokine production per se, but by the production of potentially counter-protective cytokines. We sought to maximize assay sensitivity by using semi-quantitative RT-PCR techniques to analyze cytokine mRNA extracted from lesional tissue (bone marrow). In preliminary data we provide evidence that IL-lO mRNA is present at relatively high levels in active kala-azar, being at or below the limits of detectability after treatment, and absent in uninfected controls. The cellular source of this potentially down-regulatory cytokine is being investigated in patients from India by repeating the above experiments using bone marrow cells positively and negatively selected for B cells, T cells, or monocytes using magnetic beads. In in vitro macrophage infections, procyclics are killed by macrophage several hours post infection whereas metacyclics survive, transform to amastigotes and replicate. These different outcomes provide a good model for studying cellular responses elicited during infection that are associated with either intracellular killing or survival. L. major promastigotes induced remarkably weak cytokine responses by mouse macrophages during the early stage of infection and no dramatic differences were observed in the induction of cytokine mRNA expression by procyclic and metacyclic promastigotes. Both procyclic and metacyclic promastigotes downregulated cytokine gene expression by LPS-treated macrophages. This downregulation was more efficient in the case of metacyclics and this could be related to their survival. Significant differences were observed between procyclics and metacyclics in the activation of the oxidative burst. It seems likely that the oxidative burst is the major mechanism involved in the killing of procyclic promastigotes. These results suggest that activation pathways leading to oxygen-dependent and independent killing mechanisms can be triggered independently in Leishmania infections.